Virtual image correction lens system

ABSTRACT

A visual system including a viewing window to present to a viewer a realistic virtual image representing a plurality of objects is disclosed. The images of the plurality of objects are often projected onto flat surfaces, and this disclosure sets forth a lens system which corrects for the spherical aberration caused by the spherical lenses and for the changes in the position of the viewer&#39;&#39;s eye by providing a window having an elliptical lens and a spherical corrector.

Inventors Thomas P. l

Baltimore, Md.; Walter E. Myles, Alexandria, Va. Appl. No. 810,859 FiledSept. 17, 1968 Division of Ser. No. 511,149, Dec. 2, I965. Patented Apr.20, 1971 Assignee Singer-General Precision Inc. Binghamton, N.Y.

VIRTUAL IMAGE CORRECTION LENS SYSTEM 2 Claims, 3 Drawing Figs.

U.S.Cl 350/189,

350/175 Int. Cl G021) 3/04 FieldolSearch 350/189,

175 (E), 198, (inquired) JV emu-M Cited UNITED STATES PATENTS 1,507,2129/1924 Silberstein 350/189 3,459,468 8/1969 Marx et al 350/ 1 89XFOREIGN PATENTS 395 1900 Great Britain 350/189 Primary Examiner-DavidSchonberg Assistant Examiner-Paul A. Sacher Attorneys-Francis L.Masselle and William Grobman ABSTRACT: A visual system including aviewing window to present to a viewer a realistic virtual imagerepresenting a plurality of objects is disclosed. The images of theplurality of objects are often projected onto flat surfaces, and thisdisclosure sets forth a lens system which corrects for the sphericalaberration caused by the spherical lenses and for the changes in theposition of the viewers eye by providing a window having an ellipticallens and a spherical cot-rector.

FIG; 3

INVENTOR. Thomas P. Neuberger Walter E. Myles VIRTUAL IMAGE CORRECTIONLENS SYSTEM This application is a division of the copending US. Pat.application Ser. No. 5ll,l49, filed on Dec. 2, l965 in'the names ofThonms P. Neuberger and Walter E. Myles and entitled Virtual WindowDisplay.

This invention relates to displays and, more particularly, to displaysof virtual images which are composites of two or more separate images.

In the training of pilots and passengers for travel throughinterplanetary space, the very nature of the mission itself indicatesthe necessity of simulated flights for training purposes. The problemsof landing a craft upon a distant celestial body or of rendezvousing inspace with other vehicles will be at least partially accomplished by thepilot's reaction to visual observations made through windows providedfor this purpose. Suitable training in simulated vehicles infers propersimulation of the visible stimulations to the pilot as well as blindstimulations. To provide such suitable simulations of visible phenomena,the displays must be capable of simulating relative movement of a closeobject with respect to a more distant background. In the properresponses are evoked only by displays which are realistic in appearanceas well as in movement of both the simulated objects and the observer.

The simulated display must incorporate a background which appears to beat an infinite distance from the observer and which must yet change inappearance with the progress of the simulated mission; and it must alsoincorporate images representative of objects relatively close to theobserver, which objects not only may change in appearance as thesimulated mission progesses, but which may also vary in distance fromthe observer as time moves on. ln addition, the close objects must beviewable from any of several different angles and at each must present abelievable and realistic image. Thus, as a simulated mission progresses,the display must vary also, and in a wholly believable fashion.

There are several thing which contribute to the believability of adisplay. One is the fidelity with which the relative distancesof objectsof varying distance from the observer is simulated. Another is theelimination of aberration which normally is present in optical systems,and especially in displays which may include the projection of images onflat surfaces or from flat surfaces. Another factor is the ability ofthe display to appear natural from any aspect from which it is viewed bythe observer. It must appear to move realistically or to stand still asthe observer moves his head.

Plausibility is important in instilling in the mind of a trainee in asimulated situation the impression that he is really taking part in anactual mission. The closer to the actual situation he believes himselfto be, the more effective will be both his responses and the actualtraining itself. One important aspect of a display is the depthimpression created. To achieve an appearance of reality, objects closeto the observer must appear to be close, and objects distant must appearto be distant. When celestial objects are portrayed, their distance froman observer must appear to be infinite, particularly with respect toother space vehicles, and the like. One manner in which this particularaspect of a display may be created is to provide the appearance ofobjects which are supposed to be at differing distances from theobserver with differing relative movements with' time. Thus, as amission progresses, the appearance of a fixed star background willchange; different constellations will be presented for example; but thechanges of the star background will be at a much slower rate than thechanges in the appearance of closer objects. Thus, an approaching objectwill appear to approach against an apparently unchanging backgroundwhile, actually, the background is also changing, but at a slower rate.

Distortion in the presentation of a display probably does more todestroy the appearance of reality than anything else. In fact, theelimination of distortion in images is the subject of much research.There are two general types of distortion which are of primaryimportance in imagery. They are color aberration and sphericalaberration. Chromatic aberration is 75 49 is movable.

the focusing of the various colors at difi'erent points, producing aspectrum spread; spherical aberration is the deviation of light raysfrom an expected path with respectto adjacent light rays. A display in asimulated training device must be corrected for optical aberrations.Unfortunately, in the past optical correction systems have been complexand expensive and have usually resulted in dim imagery due to theabsorption of much of the light being transmitted.

Parallax is present in virtually all objects which are viewed. Parallaxis the apparent displacement of an object being viewed as the viewerchanges his position with respect to that object. This is particularlytrue with close objects viewed against a distant background. Thus, if adisplay is to depict a close object against a fixed star background, theclose object must appear to move more than the stars as the viewer moveshis head. This is one aspect of simulated displays which has been themost overlooked, and yet, it is one of the most important whenplausibility is to be achieved. Obviously, if an observer moves his headto the right, and the moon in a display appears to rapidly move to theleft, the impression of reality is destroyed.

It is an object of this invention to provide a new and improved displaysystem.

It is another object of this invention to provide a new and improvedlens system for creating credible composite displays of the images ofseveral objects.

It is a further object of this invention to provide a new and improveddisplay system in which at least a portion of the optical system removesspherical aberrations of the lens.

It is still another object of this invention to provide an opticalsystem which presents to viewers realistic images which are projectedonto flat surfaces.

It is still a further object of this invention to provide a virtualimage window display in which spherical aberration, curvature of thefield, and distortion due to viewer displacement from the optical axisare eliminated.

Other objects and advantages of this invention will become more apparentas the description proceeds, which description should be consideredtogether with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a system for creating a compositedisplay according to this invention; and

FIGS. 2 and 3 are schematic views of lenses which may be used in thesystem of this invention.

The principles of the display system of this invention can be explainedwith reference to FIG. 1 in which two separate optical systems 30 and 50are illustrated. Consider first the system 30 in which an imagegenerator 31 in the form of a housing containing a light source (notshown) and a lens system 29 having an optical axis 10 transmits an imageto the positive lens 13. The lens 13 lies on the optical axis 10. Facingthe lens 13 on the axis 10 is a duplicate lens 17. Midway between thelenses 13 and 17 and at the focal point 15 is a semitransparent screen23 upon which the image transmitted through the lens 13 is projected. Atransparency is mounted in the housing 31, and the light passes from thesource through the transparency and the lens system 29 to provide afield having an angular width .4. The second lens system 50 comprises animage generator 46 which is similar to the generator 31 and includes alens system 44. A positive lens 47 similar to the lenses l3 and 17 ispositioned before the generator 46. A semitransparent screen 49 islocated on the optical axis 45 of the lens 48 at about the focal pointof that lens. A light splitter, or semitransparent mirror 37, iscentered about the intersection of the optical axis 10 of the lens 13and the optical axis 45 of the lens 48 and forms an angle of 45 withboth of these axes. The mirror 37 is so located with respect to thelenses [3, l7 and 48 that the distance between the lenses l7 and 48along the optical axis 45 bent by the mirror 37 may be equal to thedistance along the optical axis 10 between lenses l3 and 17. Screen 23is stationary; screen The light from the source in the housing 31 passesthrough the transparency in the housing and through the lens system 29to the lens 13. The image field through the transparency and through thelens 29 has an angular width A and is limited by the dimensioning of theparts. The lens 29 serves as an image stop. The light coming from thegenerator 31 defining the desired image is designated by light rays 34and 36 and forms, on the screen 23, the desired image. Screen 23 is arear projection, or semitransparent screen, and the image formed thereonis also picked up by the lens 17. Both lenses 13 and 17 are located afocal length away from the screen 23. A viewer peering through lens 17looks through an object stop 55 and sees the desired image with thedistortions introduced by the lens 13 corrected. The field of viewpassing through the stop 55 covers an angle A, which is equal to thewidth of the field of view from the generator 31.

In the meantime, the generator 46 is operating in a manner similar tothat of the generator 31. Light from a source (not shown) pases througha transparency (also not shown), both of which are contained in thehousing 46, and through the lens system 44 to project through the lens48 the desired image from the transparency. Since the generator 46 issimilar to the generator 31, the angular width of the field therefrom isthe same as that from generator 31. The image passes through the lens 48with the opening in the generator 46 serving as a stop, and the image isformed on the screen 49. The screen 49 is movable as contrasted with thescreen 23 which is fixed in position. The light which defines the imageon the screen 49 passes to the semitransparent mirror 37 and isreflected through the lens 17 toward the observer. The image from thescreen 23 also passes through the mirror 37, and the two images aresuperimposed there. It has been found that lenses 13 and 48 may besimilar to, but not necessarily identical to, the lens 17 and stillprovide correction of the distortion to produce a believable image. inaddition, the lenses 13 and 48 may be smaller than the lens 17,resulting in a material saving in initial cost. Further, the lens 48 maybe adjusted to cooperate with the lens system 44 of the generator 46 sothat the distance from lens 48 to the screen 49 is different from thefocal distance of the lens 17 The projector lens system 44 is arrangedso that its distance from the transparency is less than that lenssystem's infinity focal length. Then the distance between lens 48 andscreen 49 may be increased. In this manner, the overall system may beadjusted to accommodate different housing requirements.

One of the important features of a display system of this type is theability of the system to form a plausible composite image of two or moreseparate images of objects which are at different distances from theobserver. The image produced by the generator 31 is at the greatestdistance from the observer. In fact, for this discussion, consider theimage produced by the generator 31 to be a fixed star background whichis essentially at infinity. The image produced by the generator 46 isone of an object much closer to the observer. To approach reality, thetwo images must be combined to form a single image in which the far andthe near objects appear to act differently. This is the purpose forfixedly supporting screen 23 and having screen 49 movable. In systems ofthis nature, the following equation holds true:

Where F is the focal length of the lens D,is the distance from the lensto the object, and

D, is the distance from the lens to the image. In the case of the screen23, the image formed on that screen by the lens 13 forms the object ofthe lens 17. Since the screen 23 is located at the focal distance fromthe lens 17, 1/F=l/D,,, and l/D,=0. Then, D, is infinite. Thus, theimage which is passed through the lens 17 from the screen 23 appears tothe observer to be an infinite distance away. In the system 50, thescreen 49 is movable from the focal point of the lens 17 in thedirection toward the lens 17. Moving screen 49 away from lens 17 throwsthe image out of focus. This means that in the system 50, the image theobserver views appears to be at a finite distance from the lens 17. Therelative distance can be adjusted over a range by the positioning of thescreen 49. In addition, the screen 49 can be automatically movable inresponse to a prescribed condition so that the apparent distance of theimage, produced by the generator 46, from the observer varies with thecondition. When the screen 23 is at the focal point of the lens 17,light which is passed from the screen 23 to the lens 17 is divergent,but the light which emerges from the lens 17 is collimated. Should thescreen be less than the focal distance from the lens, the light emergingfrom the lens is divergent. In either case, the image seen by the vieweris a virtual image.

The lenses 13, 17, and 48, are used to project an image produced by animage generator onto a flat screen or to view that image. To reduce theeffects of spherical aberration as much as possible, an ellipticalwindow lens has been developed to flatten the image. One form of such alens is shown in FIG. 2 in which an ellipse 102 is shown in dashed lineswith one end of the ellipse 102 formed with solid lines to define aplanoconvex lens 101. A screen 103 is shown approximately at the focalpoint 104 of the lens 101. The radius of curvature of the lens 101 iscontinuously changing along its curved surface. Thus, the radius ofcurvature shown by line 105 at the outer edge of the lens 101 is longerthan the radius of curvature 106 at the center of the lens. By the sametoken, the distances from the convex side of the lens 101 to the surfaceof the screen 103 are not the same from all points, and, for example,the line 107 is longer than the line 108. The relationship between thelength of the line 108 with respect to the length of the line 106 is thesame as the relationship of the length of the line 107 with respect tothe line 105. In this manner, the elliptical lens 101 tends to flattenan image transmitted through it.

Another way in which distortion can be introduced into the system toreduce the reality of the final results is by the swimming effect whichis often produced when the observer moves his eyes from the optical axisof the window lens 17. To reduce this swimming effect, a sphericalcorrector may be used as shown in FIG. 3. A window lens 111, of theelliptical type shown in FIG. 2, projects an image which passes througha semitransparent mirror 116 through a window 112 where it is viewed byan observer whose eye 121 is on the optical axis of the lens 111. Thewindow 112 is mounted in a bulkhead 123. Between the window 112 and thelens 111 is a spherical corrector 113 held in place by a bezel 124 oranother suitable device. The diameter or diagonal of the window 112 isabout the same size as the diameter of the spherical lens 113. Light, asshown by lines 117 and 118, which passes through the lens 111 and thecorrector 113 to an observer on the optical axis 115 is not affected bythe corrector 113. However, that light, as shown by the line 119, whichpasses through the lens 111 and the corrector 113 toward an observer 122at a point off the optical axis 115, is refracted by the corrector 113producing a negative aberration, tending to compensate for the apparentmovement of the image as the eye moves from the position occupied at 121to the position 122. The farther off the optical axis the observer is,the greater the correction. In addition, the system shown in FIG. 3demonstrates how, with a lens 111 which is larger than the window 112, awide field of view which is actually larger than that seen at any onetime through the window 112 is achieved. As an observer moves his headto look through another portion of the window 112, he looks through thewindow at a different angle seeing more of the image than that availableto him when his eye is on the optical axis 115. A shroud 125 surroundingthe window 112 and the lens 111 prevents looking beyond the lens 111. Inthis manner, a 90 field of view is achieved, with the angle of viewthrough the window 112 at any point being less than that.

In summary, the system of this invention provides a virtual image windowdisplay which approaches reality and which is relatively inexpensive inboth its construction and its maintenance. The system of this inventionutilizes image splitters or semitransparent mirrors for combining imagesonto a single optical axis. It utilizes simple positive lenses which areself-correcting. it utilizes two or more translucent screens upon whichimages may be projected and which are used to create the impression ofdiffering distances from an observer of the several images generated bythe system. Simple and inexpensive image generators of any suitable typemay be used, and used interchangeably, to produce the images and theprogrammed changes of those images to meet the needs of any particularsituation. The image transmission and projection systems used in thesystem of this invention are kept simple and direct to reduce as much aspossible the absorption of light and the subsequent dimming of theimages. In addition, the projection systems used are low in cost andinherently self-correcting to reduce the overall cost of the system andthe'cost of producing a realistic image. The final results are realisticin their appearance and in their actions. Distant objects appear to beat a distance; close objects appear to be close.

While it is realized that the above specification may indicate to othersin the art additional ways in which the principles of this invention maybe used, it is intended that this invention be limited only by the scopeof the appended claims.

We claim:

1. A window lens system for presenting to an observer a realisticvirtual image, said system comprising a flat surface upon which an imageis focused, a planoconvex lens arranged with its convex surface facingsaid flat surface, said convex surface of said lens having a graduallyincreasing radius of curvature from the optical axis of said lens sothat said lens presents to an observer looking at the plane surface ofsaid lens a virtual image which is in focus throughout its extent of theimage formed on said flat surface and a spherical connector foreliminating swimming effects of an image viewed through said window asthe observer moves from the optical axis, said corrector comprising aportion of a hollow sphere having a substantially uniform wallthickness, said sphere being fonned of an optically transparentmaterial, and means for mounting said corrector in said window with itsconcave surface toward the observer.

2. The system defined in claim I wherein said flat surface is locatedapproximately at the focus of said lens and wherein the light from saidflat surface to said lens is divergent and from said lens to saidobserver is collimated.

1. A window lens system for presenting to an observer a realisticvirtual image, said system comprising a flat surface upon which an imageis focused, a planoconvex lens arranged with its convex surface facingsaid flat surface, said convex surface of said lens having a graduallyincreasing radius of curvature from the optical axis of said lens sothat said lens presents to an observer looking at the plane surface ofsaid lens a virtual image which is in focus throughout its extent of theimage formed on said flat surface and a spherical connector foreliminating swimming effects of an image viewed through said window asthe observer moves from the optical axis, said corrector comprising aportion of a hollow sphere having a substantially uniform wallthickness, said sphere being formed of an optically transparentmaterial, and means for mounting said corrector in said window with itsconcave surface toward the observer.
 2. The system defined in claim 1wherein said flat surface is located approximately at the focus of saidlens and wherein the light from said flat surface to said lens isdivergent and from said lens to said observer is collimated.